Is your feeder near a bush the birds can dive into for cover, and is it in a spot where it is perfect for the hawk to get a surprise attack in. i watched one take a bird from a feeder when I was high up and you could see the prey had no chance until the Hawk was right on top of it as it came between the houses.
Thanks - that's more or less what we've done. The centre lawn feeders are about 5m from a large bush where the small birds gather to dash out to feed and then come back. However, we've seen the sparrowhawk dive straight down onto these feeders and grab a bird in one clean movement.
So, we've switched to using only the feeders hanging by the kitchen window, which have a fig tree bush right by them for cover. This hasn't stopped the sparrowhawk having a go but being so close to a wall, the wall is preventing a clean dive down, plus she can't get into the bush very easily (she has tried!). As she is less successful, she's now having a go at the blackbirds on the lawn.